A microchannel plate (MCP) is a planar component used for detection of particles that cross the boundary of its surface to enter one of thousands if not millions of hollow channels distributed across the MCP. Each channel is an electron multiplier that produces an electrical current generated by the multiplication of electrons via secondary emission. The currents from respective channels of the MCP emerge as localized streams of electrons that, unlike other electron multipliers, retain a spatial distribution of the particle impingement patterns across its surface. For this reason, MCPs are widely used in image intensifiers.
In a typical image intensifier configuration, a photocathode, an assembly of one or more MCPs, and a cathodoluminescent element, such as a phosphor screen, are enclosed within a vacuum. An electron is generated from an impinging photon by the photocathode and is multiplied by the MCPs, and the electrons that emerge from the MCPs are converted into photons by the phosphor screen. The photocathode is constructed from a wavelength-selective material, typically in a very thin layer, that is exposed in the chamber of the device. A major drawback of these types of image intensifying devices is that the electrostatic fields that transport the electrons from the photocathode coating to the MCP assembly also transport positive ions generated in the electron multiplication back towards the photocathode. Because these positive ions may have considerable mass, irreparable damage is done when such an ion strikes the photocathode. Efforts to mitigate this ionic transport are ongoing in the MCP field.
Depositing a thin ion barrier film (IBF) on the input side of the MCP is a conventional technique by which ions are prohibited from reaching the photocathode. There are several drawbacks to the use of the ion barrier film, one of which is a reduction in the signal-to-noise ratio (SNR) owing to absorption of electrons by the ion barrier film. Another drawback is the formation of a halo around objects in the image due to photoelectrons being incapable of initially penetrating the IBF and instead bouncing to another location and penetrating there. Yet another drawback is that higher voltage must be applied between the photocathode and the MCP in order to overcome the electron barrier established by the IBF.
Despite the recognized advantages of using ion barrier films, particularly where the useable lifetime of the photocathode is extended, poor imaging performance continues to frustrate consumers and designers alike.